The following discussion is not an admission that anything discussed below is citable as prior art or common general knowledge.
Seawater may be desalinated by reverse osmosis. Water is taken from the ocean through an intake system into a treatment plant. The raw seawater is pre-treated, for example to remove particles and organic foulants. The pre-treated water then passes through a reverse osmosis system. A portion of the seawater permeates through one or more stages of membranes and is used, for example, as municipal drinking water.
Another portion of the seawater is retained or rejected by the membranes. This reject stream has an increased salt content and osmotic pressure. This increase in osmotic pressure limits the extent to which the reject stream can be concentrated, and therefore limits the recovery of the process. Most seawater reverse osmosis plants are designed to run at about 35-50% recovery. In some instances, for example with very high pressure operation or additional scaling control steps, higher recovery rates of 55-60% are possible.
Scaling may also limit the maximum concentration of the reject stream in reverse osmosis and other types of desalination systems. For example, thermal desalination devices such a evaporators are typically limited to recovery rates of about 10-40%, primarily due to scaling problems. Calcium sulfate in particular becomes less soluble as the temperature increases and problems with calcium sulfate scale may determine the maximum operating temperature and recovery rate.
Due to the limited recovery of a desalination plant, the intake and pre-treatment systems handle significantly more than the output of potable water. For example, a 50 MGD seawater reverse osmosis (SWRO) desalination system operating at 40% recovery requires intake and pre-treatment systems sized for about 125 MGD. Each of the intake and pre-treatment systems may be as expensive as the reverse osmosis system itself. The intake and pretreatment systems also consume power and chemicals. Further, withdrawing excess seawater increases the ecological damage to sea life caused by the desalination plant.